that's a nice mosin you have there, I'm glad you decided against the cheaper ATI alternatives for scope base.
however your plans for a piston driven mosin nagant semi is going to be a near impossible task with a lot of things to overcome first.
1. the magazine is a large obstacle. 5 round capacity magazines are pretty limiting when it comes to a semi auto, with the frequency of reloads there is little advantage over a bolt action. there are some people working on a 10 round magazine but it is still in the prototype stage. promag has a 10 round detachable magazine but it must be incorporated into the new Archangel polymer mosin stock to work which would add it's own challenges.
2. you would have to completely design your own bolt, a lot of precises machinery will be necessary for this to happen.
3. semi autos are not so forgiving of crude finishes and heavy tooling marks. you would have to go in and buff out a lot of the ridges, and bumps to help streamline the action.
4. where would the piston go? you can't go over the op as that's where your sights are located, if you go to either side you'll mess with the balance of the rifle and it will always want to rotate to one side and cause a fall in accuracy. going through the bottom would be your only option in this case which would bring you back to having to find a way to incorporate it into the stock without impeding the magazine.
what you are attempting to do has been done before but not in the way you plan to do it. during world war one a conversion known as the Pedersen Device was used to convert springfield 1903s into semi automatic rifles. this design was blow back, completely operated by recoil, no piston necessary. however as 3006 is way too powerful to put in a blowback design, they invented a small 30 caliber pistol cartridge to work with the device. as these smaller rounds could not feed in the magazines, a new 40 round top feed magazine was created. after all this was done, then they had to actually design a semi automatic bolt to work with all of these modifications. all of this was a colossal undertaking for remington arms even in a factory setting and they were working with a base design that that was much more refined than the 91/30.
the challenges of doing all this are just too astronomical for anyone that doesn't already own a firearms factory. for the thousands of dollars and countless hours that would be required drafting schematics and actually attempting to fabricate a prototype, it would just be way cheaper and easier to go out and buy a russian SVT40 in 7.62x54R that is piston driven and has a 10 round detachable mag and call it a day.
all guns lost in a tragic smelting accident.
I never said half the crap people said I did-Albert Einstein
You can't believe everything you read on the internet-Benjamin Franklin
Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.